Chopped Intermediate Frequency Wireless Receive

ABSTRACT

A chopped intermediate frequency (IF) wireless receiver is disclosed. The wireless receiver includes a local oscillator (LO), a first and a second mixers, an LO frequency control module, an IF filter, a digital down converter and a down conversion controller. The LO provides a local oscillating signal to the first and second mixers. The first and second mixers converts a received radio frequency signal to an in-phase IF signal and a quadrature IF signal, respectively. The LO frequency control module alternately down converts a channel frequency by changing an oscillation frequency of the LO. Coupled to the digital down converter, the down conversion controller adjusts a complex sine wave within the digital down converter while the in-phase IF signal and the quadrature IF signal are being down-converted by the digital down converter to a baseband signal.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to wireless communication in general, andin particular to wireless transceivers. Still more particularly, thepresent invention relates to a chopped intermediate frequency wirelessreceiver.

2. Description of Related Art

Unlike wireline communication environments, a wireless communicationenvironment has to accommodate a large number of users sharing differentparts of the frequency spectrum with very strong signals coexistadjacent to very weak signals. Hence, a wireless receiver should be ableto select the signal of interest while rejecting all other signals.According to their respective architecture, wireless receivers cangenerally be classified under two categories, namely, homodyne receiversand heterodyne receivers.

With a homodyne receiver, the desired signal is initially selected by abandselect filter and is subsequently amplified by a low-noiseamplifier. The desired signal is then frequency translated by a mixer toDC before other baseband operations are performed on the desired signal.With a heterodyne receiver, the desired signal is sent through receivercomponents similar to those in the homodyne receiver with the exceptionthat the desired signal is translated to an intermediate frequency (IF)when signal processing operations are performed.

For heterodyne wireless receivers, image rejection refers to the abilityto select the desired signal from the image of the desired signal spacedaway by twice the IF signal. Basically, a heterodyne wireless receivershould be able to select the desired signal from its image. Otherwise,the subsequent detector circuit within the heterodyne wireless receiverwill not be able to distinguish between the desired signal and the imagesignal, and the output becomes the superposition of both signals.Accordingly, image rejection is one of the problems that are faced bydesigners of heterodyne wireless receivers.

Also, in many wireless receiver designs, all internally used oscillatorfrequencies are derived from a single high-accuracy referenceoscillator, which can produce interference at fundamental, harmonic andsub-harmonic frequencies of the reference oscillator. The mixing of suchinterference frequencies with unwanted input signals is referred to as aspurious signal. Spurious signals are commonly found in wirelessreceivers. A wireless receiver may down convert a spurious signal thatcan interfere with the desired signal of interest, and as a result, thedesired signal of interest can be corrupted.

The present disclosure provides a method and apparatus for alleviatingthe above-mentioned problems within a wireless receiver.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, awireless receiver includes a local oscillator (LO), a first and secondmixers, an LO frequency control module, an intermediate frequency (IF)filter, a digital down converter and a down conversion controller. TheLO provides a local oscillating signal to the first and second mixers.The first and second mixers converts a received radio frequency signalto an in-phase IF signal and a quadrature IF signal, respectively. TheLO frequency control module alternately down converts a channelfrequency by changing an oscillation frequency of the LO. Coupled to thedigital down converter, the down conversion controller adjusts a complexsine wave within the digital down converter while the in-phase IF signaland the quadrature IF signal are being down-converted by the digitaldown converter to a baseband signal.

Alternatively, a wireless receiver includes an LO, a first and a secondmixers, an LO frequency control module, an IF filter, a switch and adigital down converter. The LO provides a local oscillating signal tothe first and second mixers. The first and second mixers converts areceived radio frequency signal to an in-phase IF signal and aquadrature IF signal, respectively. The LO frequency control modulealternately down converts a channel frequency by changing an oscillationfrequency of the LO. Coupled to the IF filter, the switch alternatelyswaps signals paths of the in-phase IF signal and the quadrature IFsignal on a frame-by-frame basis in synchronization with the LOfrequency control module. The digital down converter subsequentlydown-converts the in-phase IF signal and the quadrature IF signal to abaseband signal.

All features and advantages of the present invention will becomeapparent in the following detailed written description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself, as well as a preferred mode of use, furtherobjects, and advantages thereof, will best be understood by reference tothe following detailed description of an illustrative embodiment whenread in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a chopped intermediate frequency wirelessreceiver, in accordance with a preferred embodiment of the presentinvention;

FIG. 2 is a block diagram of a chopped intermediate frequency wirelessreceiver, in accordance with an alternative embodiment of the presentinvention;

FIGS. 3 a-3 b graphically illustrate the potential improvements in theinterference performance of a chopped intermediate frequency wirelessreceiver with respective to image rejection; and

FIGS. 4 a-4 d graphically illustrate the potential improvements in theblocking performance of a chopped intermediate frequency wirelessreceiver with respective to spurious signals.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings and in particular to FIG. 1, there isdepicted a block diagram of a chopped intermediate frequency (IF) radiofrequency (RF) receiver, in accordance with a preferred embodiment ofthe present invention. As shown, a RF receiver 10 includes an antenna11, a low-noise amplifier 12, a pair of RF mixers 13 a-13 b, an IFfilter 15, and analog-to-digital converter (ADC) 16 and a digital downconverter 18 along with additional receiver circuitry (not shown) thatare well-known to those skilled in the art. In addition, RF receiver 10includes a radio frequency local oscillator (RFLO) 14 coupled to RFmixers 13 a-13 b for generating an in-phase signal and a quadraturesignal. RF receiver 10 also includes an LO frequency control module 19coupled to RFLO 14 for controlling the oscillation frequency of RFLO 14.Similarly, a down conversion controller 30 is coupled to a digitalcomplex sinusoid signal IFLO 17 for controlling the oscillationfrequency of IFLO 17 that is connected to digital down converter.

RF signals are initially received by antenna 11. After passing throughlow-noise amplifier 12, the RF signals enter an in-phase path 2 and aquadrature path 3 via a junction 4. A first input of RF mixer 13 a isconnected to in-phase path 2, and a second input of RF mixer 13 a isconnected to an in-phase output of RFLO 14. The output of RF mixer 13 aprovides in-phase IF signals to IF filter 15 via an in-phase signal pathI. Similarly, a first input of RF mixer 13 b is connected to quadraturepath 3, and a second input of RF mixer 13 b is connected to a 90°out-of-phase output of RFLO 14. The output of RF mixer 13 b providesquadrature IF signals to IF filter 15 via a quadrature signal path Q.Both IF signal paths for RF receiver 10 may be approximately 200 kHz.

In the embodiment as shown in FIG. 1, the IF signals within in-phasesignal path I and quadrature signal path Q of RF receiver 10 arecompletely independent from each other until the final down conversionto a baseband signal. In other words, each of the IF signal paths I andQ does not have a complex transfer function. Because the transferfunctions for the IF signal paths I and Q are real, the desired signalcan be down converted to a +200 kHz IF signal or a −200 kHz IF signal bychanging the oscillation frequency of RFLO 14.

Under time-division multiple access (TDMA), data are typicallyinterleaved several times and are collected in TDMA frames. Thus, theabove-mentioned IF signals down conversion can be performed on aframe-by-frame basis by simply utilizing LO frequency control module 19to control the oscillation frequency of RFLO 14. Preferably, theoscillation frequency of RFLO 14 is modified as follows:even frame: f _(RFLO) =f _(CH) −f _(IF)odd frame: f _(RFLO) =f _(CH) +f _(IF)Alternatively, the oscillation frequency of RFLO 14 can be modified asfollows:even frame: f _(RFLO) =f _(CH) +f _(IF)odd frame: f _(RFLO) =f _(CH) −f _(IF)where

-   -   f_(RFLO)=oscillation frequency of RFLO 14    -   f_(CH)=frequency of channel    -   f_(IF)=frequency of IF signals        Basically, the IF signal down conversions are performed as a        complex sinusoidal multiplication of the RF signals as follows:        IF(t)=RF(t)×e ^(−2xf) ^(RFLO)        where f_(RFLO)=f_(CH)±f_(IF)        No adjustment or modification to the IF signal paths I and Q        within RF receiver 10 is required.

The final down conversion of the IF signals to a baseband signal can beperformed by one of the following two methods. The first method is byadjusting the digital complex sine wave used in the final downconversation within a digital down converter such as digital downconverter 18. Specifically, the digital complex sine wave used indigital down converter 18 is adjusted by adjusting the frequency of IFLO17 via down conversion controller 30. Preferably, the adjustment isperformed as follows:even frame: IFLO(t)=e^(−jω) ^(IF) ^(t)odd frame: IFLO(t)=e^(+jω) ^(IF) ^(t)Alternatively,even frame: IFLO(t)=e^(+jω) ^(IF) ^(t)odd frame: IFLO(t)=e^(−jω) ^(IF) ^(t)where e ^(−jω) _(IF) ^(t)=Cosω_(IF) t−jSinω_(IF) te ^(+jω) _(IF) ^(t)=Cosω_(IF) t+jSinω_(IF) tω_(IF)=2τf _(IF)No adjustment or modification to the IF signal paths I and Q within RFreceiver 10 is required.

The second method for performing down-conversion of IF signals is to usea switch to swap the two IF signal paths (from +200 kHz to −200 kHz)before the down-conversion stage. The second method is furtherillustrated in FIG. 2.

With reference now to FIG. 2, there is depicted a block diagram of achopped IF RF receiver, in accordance with an alternative embodiment ofthe present invention. As shown, a RF receiver 20 includes an antenna21, a low-noise amplifier 22, a pair of RF mixers 23 a-23 b, an IFfilter 25, an ADC 26, a switch 27 and a digital down converter 28 alongwith additional receiver circuitry (not shown) that are well-known tothose skilled in the art. In addition, RF receiver 20 includes a RFLO 24coupled to RF mixers 23 a-23 b. RF receiver 20 also includes an LOfrequency control module 29 coupled to RFLO 24 for controlling theoscillation frequency of RFLO 24.

In essence, the basic functional components of RF receiver 20 aresimilar to those of RF receiver 10 from FIG. 1. The main differencebetween RF receiver 20 and RF receiver 10 from FIG. 1 is that instead ofusing a IFLO and a down conversion controller to adjust the digitalcomplex sine wave used in a digital down converter, a switch, such asswitch 27, is used to swap every other frame between the IF signal pathsI and Q. For example, in switch 27 of FIG. 2,

-   -   even frame: node a connects to node b; node c connects to node d    -   odd frame: node a connects to node d; node c connects to node b        Alternatively,    -   even frame: node a connects to node d; node c connects to node b    -   odd frame: node a connects to node b; node c connects to node d        In both of the above-mentioned cases, nodes a and b are        connected to the IF signal path I, and nodes c and d are        connected to the IF signal path Q. Although switch 27 is shown        to be located between ADC 26 and digital down converter 28 in        FIG. 2, switch 27 can be located anywhere within the IF signal        paths. For example, switch 27 can be located between IF filter        25 and ADC 26 or between RF mixers 23 a-23 b and IF filter 25.

Referring now to FIGS. 3 a-3 b, there are graphically illustrated thepotential improvement in the interference performance with respective toimage rejection within a chopped IF wireless receiver, such as RFreceiver 10 in FIG. 1 and RF receiver 20 in FIG. 2, by swapping IFsignal frequencies. FIG. 3 a represents even numbered frames (i.e., +200kHz IF signals), and FIG. 3 b represents odd numbered frames (i.e., −200kHz IF signals). As shown in FIG. 3 a, because of the limited imagerejection, a −400 kHz reference interferer 31 images into the +200 kHzside as an interference signal 32 on the even frames. As a result,interference signal 32 interferes with a desired signal 33. In FIG. 3 b,the −400 kHz reference interferer 35 does not image into a desiredsignal 34 on the odd frames. Because of the present invention, the oddframes will not “see” the mirrored interference for a −400 kHz referenceinterferer. Hence, the overall carrier/interference (C/I) ratio isimproved by approximately 3 dB since only half of the frames areaffected by the mirrored reference interference.

With reference now to FIGS. 4 a-4 b, there is graphically illustratedthe potential improvement in the blocking performance with respective tospurious response within a chopped IF wireless receiver, such as RFreceiver 10 in FIG. 1 and RF receiver 20 in FIG. 2, by swapping IFsignal frequencies. As shown, FIG. 4 a represents a RFLO signal f_(RFLO)and a spurious response f_(spur), and FIG. 4 b represents a desiredsignal 41 and an undesired signal 42. FIG. 4 c represents even numberedframes (i.e.,+200 kHz IF signals), and FIG. 4 d represents odd numberedframes (i.e., −200 kHz IF signals). During the operation of a wirelessreceiver, RFLO signal f_(RFLO) mixes with desired signal 41 to producedesired signals 44 and 45 in FIGS. 4 c and 4 d, respectively. Similarly,spurious response f_(spur) mixes with undesired signal 42 to producespurious signals 43 and 46 in FIGS. 4 c and 4 d, respectively.

Typically, the spurious response of a wireless receiver does not changewhen the frequency of a RFLO is adjusted. Therefore, as the frequency ofthe RFLO is adjusted on a frame-by-frame basis (i.e., swapping the IFsignal from +200 kHz to −200 kHz), the spurious signal down-converted toIF will be either a +200 kHz interference signal or a −200 kHzinterference signal. Either way, as the signal is swapped between +200kHz and −200 kHz, alternate frames will not “see” the wanted signaldegraded by the spurious signal. Hence, spurious signal 42 mixes intothe +200 kHz side on even frames and interferes with desired signal 44;but spurious signal 42 does not mix into desired signal 45 on the oddframes. As a result, the overall signal quality of the wireless receiveris improved because of the present invention.

As has been described, the present invention provides a chopped IFwireless receiver. The present invention improves the quality of asignal within a wireless receiver by swapping the IF signal frequency ona frame-by-frame basis. One main advantage of the present invention isthat no adjustment to the IF signal paths I and Q is required for theswapping of the IF signal frequency.

The method and apparatus of the present invention are applicable to RFreceivers suitable to be used in TDMA communication networks, such asGlobal System for Mobile communications (GSM) networks. Although a RFreceiver is used to illustrate the present invention, it is understoodby those skilled in the art that the present invention is alsoapplicable to the receiver portion of wireless transceivers.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madewithout departing from the spirit and scope of the invention.

1-26. (canceled)
 27. A radio frequency (RF) receiver comprising: a localoscillator (LO) for generating a local oscillation signal; first andsecond mixers coupled to said LO, for converting a received W signal toan in-phase intermediate frequency (IF) signal and a quadrature IFsignal, respectively; an LO frequency control module, coupled to saidLO, for alternately down converting a channel frequency by changing anoscillation frequency of said LO; a switch, coupled to said fist andsecond mixers, for alternately swapping signals paths of said in-phaseIF signal and said quadrature IF signal in synchronization with anoscillation frequency of said LO; and a down converter, coupled to saidswitch, for down converting said in-phase IF signal and said quadratureIF signal to a base band signal.
 28. The RF receiver of claim 27,wherein said LO frequency control module alternately down-converts achannel frequency on a frame-by-frame basis.
 29. The RF receiver ofclaim 28, wherein said LO frequency control module alternatelydown-converts a channel frequency byeven frame: f _(RFLO) =f _(CH) −f _(IF)odd frame: f _(RFLO) =f _(CH) +f _(IF) wherein f_(RFLO)=said localoscillation frequency f_(CH)=said channel frequency f_(IF)=said IFsignal frequency
 30. The RF receiver of claim 29, wherein said framesare time-division multiple access (TDMA) frames.
 31. The RF receiver ofclaim 28, wherein said LO frequency control module alternatelydown-converts a channel frequency byeven frame: f _(RFLO) =f _(CH) +f _(IF)odd frame: f _(RFLO) =f _(CH) −f _(IF) wherein f_(RFLO)=said localoscillation frequency f_(CH)=said channel frequency f_(IF)=said IFsignal frequency
 32. The RF receiver of claim 31, wherein said framesare time-division multiple access (TDMA) frames.
 33. The RF receiver ofclaim 27, wherein said RF receiver further includes an IF filter. 34.The RF receiver of claim 27, wherein said RF receiver further includesan analog-to-digital converter.
 35. A method for enhancing signalquality within a radio frequency (RF) receiver, said method comprising:receiving an RF signal; alternately down-converting a channel frequencyby changing a local oscillation frequency, wherein said localoscillation frequency is utilized for converting said received RF signalto an in-phase intermediate frequency (IF) signal and a quadrature IFsignal; alternately swapping signal paths of said in-phase IF signal andsaid quadrature IF signal in synchronization with said local oscillationfrequency; and down converting said in-phase IF signal and saidquadrature IF signal to a baseband.
 36. The method of claim 35, whereinsaid alternately down-converting further includes alternatelydown-converting said in-phase IF signal and said quadrature IF signal ona frame-by-frame basis.
 37. The method of claim 36, wherein saidalternately down-converting is performed byeven frame: f _(RFLO) =f _(CH) −f _(IF)odd frame: f _(RFLO) =f _(CH) +f _(IF) wherein f_(RFLO)=said localoscillation frequency f_(CH)=said channel frequency f_(IF)=said IFsignal frequency
 38. The method of claim 38, wherein said -frames aretime-division multiple access (TDMA) fiames.
 39. The method of claim 37,wherein said alternately down-converting is performed byeven frame: f _(RFLO) =f _(CH) +f _(IF)odd frame: f _(RFLO) =f _(CH) −f _(IF) wherein f_(RFLO)=said localoscillation frequency f_(CH)=said channel frequency f_(IF)=said IFsignal frequency
 40. The method of claim 39, wherein said frames aretime-division multiple access (TDMA) frames.